Photoimageable compositions comprising hexaarylbiimidazole (HABI) compound(s) and leuco dye(s) and which are imaged using ultraviolet radiation (UV means) are known.
Additives have been reported which enhance the performance for special applications. The use of sensitizing dyes, has been reported, such as, in U.S. Pat. No. 3,563,750 to make the coatings responsive to visible light. U.S. Pat. No. 3,615,567 describes inorganic halide intensifiers in photoimageable compositions. The use of organic halogen derivatives in combination with leucodycs is described in patents issued to Photohorizons Co., such as U.S. Pat. Nos. 3,493,376, 3,042,515, and 3,042,516, and in a patent issued to DuPont, U.S. Pat. No. 4,634,657. U.S. Pat. No. 4,495,020 describes the combination of hexaarylbiimidazoles, leuco dyes and certain photoacid generators to give coatings with enhanced electrical conductivity. Most recently, in U.S. Pat. No. 5,407,783, it was disclosed that the effectiveness of UV-induced color formation of thermally stable systems could be enhanced by the addition of certain dibromo derivatives.
Systems have been described which generate an inhibitor to color-formation by exposure to light (U.S. Pat. No. 3,390,996), heat (U.S. Pat. No. 3,390,995) or light and heat (U.S. Pat. No. 3,383,212, and U.S. Pat. No. 4,332,884). In other cases, color formation was prevented by restricting the mobility of color formation, either by use of a thermoplastic binder, which required heating of the composition before, during or subsequent to UV exposure to allow color to form (U.S. Pat. No. 3,615,481). A system also described stabilization of background after color-formation as a result of a subsequent polymerization step, effected by light or heat (U.S. Pat. No. 3,615,454) which locked the color-forming components in place.
The use of photoactive halogen compounds in combination with photosensitive biimidazoles and leuco dyes in systems containing microcapsules has been described. Other modifications in which encapsulation is employed to control background and/or image stability have been reported. Thus a number of patents (U.S. Pat. Nos. 4,929,530, 4,962,009 and 4,981,769) describe systems in which color-formation is effective inside capsules and heat is employed to rupture these in order to make contact between the components of color formation with chemical fixing agents so as to stabilize the imaged areas. Here the capsules are involved in separating the imaging and fixing components and to provide a relatively stable system. All these systems require some modicum of image stabilization to prevent significant color buildup in the unimaged areas, and "add-on" of images to previously exposed image areas is difficult. No systems have been disclosed which do not undergo significant changes when viewed in ambient light, thus reducing the ability to form color effectively when re-exposed.
Without exception, all systems described above are focused on the use of ultraviolet or visible light to produce image patterns and do not contain any disclosure or even a hint thereof on the use of heat and/or near IR radiation to effect color formation.
Early work reported that color-formation of biimidazole/leuco dye systems could be achieved with other forms of radiant energy, e.g., heat or electron-beam radiation. In U.S. Pat. No. 3,585,038, it is disclosed that, only at elevated temperatures, could color-formation be achieved with HABIs and triarylmethane leuco dye salts. German Pat. No. 2 224 725, issued to Wagner Pelikan, describes compositions of unspecified HABIs and relatively easily oxidizable leuco crystal violet, which can form colored image(s) under the influence of imagewise heating. No provision for room light stabilization of either system was reported. Most significantly, there is no disclosure or even a hint thereto in either of these two patent references of significantly improved thermally imaging compositions comprising at least one acid-generating compound in addition to a HABI compound(s) and a leuco dye(s).
Thus, in sharp contrast to photoimageable compositions imageable with ultraviolet or visible radiation, effective thermally imageable compositions are not known to the art. A need for developing efficient thermally imageable compositions has been stimulated by the requirement of producing products which have sensitivity to thermal printheads and also near-IR emitting lasers. Thus there is a continuing need for improved thermally imageable compositions.